The present invention relates generally to signal processing, and more specifically to techniques for canceling acoustic echo.
Hands-free full-duplex communication systems are widely used for various applications such as speakerphone, hands-free car kit, teleconferencing system, cellular phone, and so on. Full-duplex refers to simultaneous two-way communication between a near-end user and a far-end user. In a hands-free full-duplex system, a speaker at the near-end site emits an acoustic signal from the far-end user. A microphone at the near-end site picks up an acoustic signal from the near-end user as well as a portion of the acoustic signal emitted by the speaker and reflections from the borders of an enclosure, such as a room or the interior of a car. The microphone signal is processed and sent to the far-end user, who would be annoyed at listening to his/her own voice delayed by the signal path in the system. This acoustic disturbance is referred to as echo. In certain instances, instability in the system may result in the echo causing howling, which is highly undesirable.
Echo cancellation is used in many communication systems to combat echo as well as to prevent howling. For example, echo cancellation is typically used in a hands-free full-duplex environment, such as a vehicle or a room, where the speaker and microphone may be located some distance from a user. Conventionally, echo cancellation is achieved with a circuit that employs an adaptive filter. This adaptive filter may implement a least mean square (LMS) algorithm or a normalized least mean square (NLMS) algorithm. The adaptive filter performs echo cancellation based on a reference signal, which may be a line input from a communication or telematics device such as a cellular phone or some other device. The adaptive filter is typically able to remove a portion of the echo that is correlated to the reference signal.
However, conventional echo cancellation techniques are often not able to provide good full-duplex performance. This is particularly true for echo resulting from non-linearity of circuitry in the communication systems. Such circuitry may include, e.g., the speaker, analog-to-digital converter (ADC), digital-to-analog converter (DAC), and so on, which generates echo that is not correlated to the reference signal. Good full-duplex performance is highly desirable for natural conversation between multiple parties.
As can be seen, techniques that can effectively cancel acoustic echo for good full-duplex performance in communication systems are highly desirable.